A conventional manufacturing method of an optical disk 200 having a plurality of recording faces will be explained in reference to FIGS. 25(a) to 25(c), and FIGS. 26(a) and 26(c) (see, for example, Japanese Laid-Open patent Japanese Unexamined Patent Publication No. 2001-134981 (Tokukai 2001-134981 published on May 18, 2001).
Firstly, a first substrate 100 having formed thereon recording pits 101 is formed by the injection molding method or the 2P method, and then a first reflective film 102 is formed so as to cover the first pits 101 (see FIG. 25(a)), thereby forming a recording face 201.
Next, on the first substrate 100 having formed thereon the first reflective film 102, an original substrate 104 is placed with a predetermined interval (see FIG. 25(b)). On this original substrate 104, formed are second bits 105 having recorded thereon different information from that of the first pits 101 (see FIG. 25(b)). Thereafter, a recording layer 103 is formed by filling the space between the first substrate 100 and the original substrate 104 with ultraviolet ray curing resin, and hardening the ultraviolet ray curing resin by projecting thereon an ultraviolet ray (FIG. 25 (c)).
Next, after removing the original substrate 104, a second reflective film 106 is formed on the first recording layer 103 having copied thereto the second recording pits 105 are copied, thereby forming a recording face 205.
The first substrate 100 on which the recording face 201 and the recording face 205 are formed, and a second substrate 107 on which a recording face 208 having formed thereon the third pits 108 and the third reflective film 109 is formed are placed with a predetermined interval in between so that the recording face 205 and the recording face 208 face each other (FIG. 26b). The space between the first substrate 100 and the second substrate 107 is filled with ultraviolet ray curing resin 110. Then, the recording layer 110 is hardened by projecting thereon an ultraviolet ray so as to connect the first substrate 100 and the second substrate 107 together (see FIG. 26 (c)).
In the foregoing process, an optical disk 200 including the first recording face 201 having formed thereon the first bits 101, a recording face 205 having formed thereon the second pits 105, and the recording face 208 having formed thereon the third pits 108 can be manufactured.
In the foregoing conventional example, explanations will be given through the case of the method of forming an optical disk with the three-layered recording face. However, by repeating the foregoing copying process, it is possible to form an optical disk having a greater number of recording faces.
However, the optical disk 200 formed by the foregoing manufacturing process has the following problems as will be explained below.                a) The optical disk 200 cannot be maintained flat.        b) An interval between adjacent recording faces cannot be controlled with high precision.Problem a)        
Generally, it is necessary to form the first recording layer 103 and the second recording layer 110 in a thickness of around 10 μm for the purpose of preventing an interlayer cross light or interlayer crosstalk generated when recording or reproducing.
In the foregoing method, when forming the first recording layer 103, the space between the first substrate 100 and the original substrate 104 is filled with liquid ultraviolet ray curing resin, and hardening the resin with an application of an ultraviolet ray. Here, a problem arises in that the recording layer 103 shrinks in the hardening process with an application of the ultraviolet ray.
The foregoing problem of shrinkage arises also in the process of forming the recording layer 110.
For example, in the case where the recording layer 103 and the recording layer 110 are formed in a thickness of 20 μm, the optical disk 200 is tilted to a large extent due to the shrinkage when hardening, and it becomes no longer possible to maintain the disk flat. Furthermore, when forming other recording layer 103 in addition to the recording layers 103 and 110, the optical disk 200 would be tilted to a larger extent.
When adopting the foregoing disk 200 with the foregoing problems of a large tilt which makes it difficult to maintain the disk 200 flat, coma aberration of the light beam would be increased, which makes it difficult to form a desirable light beam spot, thereby deteriorating the recording/reproducing characteristics.
Problem b)
According to the foregoing manufacturing method, in the process of filling the space between the first substrate 100 and the original substrate 104 with liquid ultraviolet ray curing resin, the original substrate 104 and the first substrate 100 are liable to be partially distorted.
The foregoing partial distortion results in uneven interval between the original substrate 104 and the first substrate 100, i.e., the thickness of the first recording layer 103.
Furthermore, in the process of connecting the substrates together as shown in FIG. 26(c), it is necessary to carry out the process of hardening the second recording layer 100 in the state different from that shown in FIG. 25(c). Namely, in the state shown in FIG. 25(c), the ultraviolet ray curing resin filled in the space between the first substrate 100 (generally made of plastics) and the original substrate 104 (generally metal plate or glass plate). In contrast, in the state shown in FIG. 26(c), the ultraviolet ray curing resin filled in the space between the first substrate 100 and the second substrate 107 (generally made of plastic) is hardened.
As described, when carrying out the process of hardening the recording layers under different conditions, such hardening conditions as a rise in temperatures when hardening, etc., are liable to change, and it is difficult to form the first recording layer 103 and the second recording layer 110 in the same thickness.
As described, in the foregoing conventional manufacturing process, the thickness of each recording layer becomes partially uneven, or the thickness between recording layers becomes uneven, which results in such problem that the interval between the adjacent recording faces cannot be controlled with high precision.
When adopting the foregoing optical disk manufactured by the conventional method, in which an interval between the recording faces varies, a spherical aberration occurs in the light beam when recording/reproducing, resulting in the problem of increasing a focused beam spot diameter or deterioration in recording/reproducing characteristics.